From 2004 to 2008, raw data was siphoned from an internet exchange point in Frankfurt and forwarded to the NSA, the Süddeutsche Zeitung newspaper and regional public broadcasters NDR and WDR reported on Friday.
The reports cited secret government documents submitted to the ongoing parliamentary inquiry into NSA spying.
It was first reported in June that the BND was handing information collected in Frankfurt to the NSA, codenamed "Eikonal," but information on German citizens was said to have been filtered out.
According to the latest Bundestag documents, however, BND internal tests showed that at least 5 percent of the German citizens' communications data could not be filtered.
An "absolute and mistake-free" separation of German and foreign citizens' communications is not possible, the secret documents said.
Frankfurt's DE-CIX internet exchange point is the largest in the world. Data streams from various internet providers meet there to be passed onto their respective destinations.
dr/jm (AFP, dpa, Reuters)